Strong adsorption of aminotriazines on graphene

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Strong adsorption of aminotriazines on graphene James D. Wuesta and Alain Rochefortw*b Received (in Cambridge, UK) 14th December 2009, Accepted 5th March 2010 First published as an Advance Article on the web 29th March 2010 DOI: 10.1039/b926286e DFT calculations reveal that aminotriazines have a strong affinity for graphite and suggest that part of the driving force for adsorption is a specific attractive interaction of NR2 groups with the underlying surface. Molecular adsorption on surfaces is a key step in processes of great scientific and technological importance, such as crystallization and heterogeneous catalysis.1 A potent tool for studying adsorption is scanning probe microscopy (SPM), which allows adsorbates to be imaged with submolecular resolution in favorable cases. By revealing directly how molecules are adsorbed and positioned relative to their neighbors, SPM helps guide the effort to engineer adlayers with defined structures and properties. Successful engineering of thin molecular films requires careful analysis of many factors, including the structure of the individual components, their interactions with one another, and their affinity for the underlying surface. For adsorption on atomically flat surfaces, molecules that can adopt planar topologies are ideal, and their 2D organization can often be controlled predictably when they engage in strong directional intermolecular interactions such as hydrogen bonds.2 Deposition from solution offers a convenient and general way to create adlayers. It avoids the need for highvacuum technology, can be used even when the adsorbates have low volatility, and allows their 2D organization to be assessed immediately at the liquid–solid interface when their affinity for the surface is sufficiently strong. Many previous studies of 2D assembly have been guided by purely qualitative evaluations of molecular topology and association. Such approaches remain useful, but the current demands of the field now require detailed quantitative analyses that measure the strength of adsorption and explain why it occurs. We recently reported calculations that probe the adsorption of substituted derivatives of benzene on graphene, which serves as a model for graphite, a common substrate for studies of 2D molecular assembly.3 Further computational studies have now revealed that aminotriazines have an unusually strong affinity for graphite and are therefore highly attractive candidates for creating 2D molecular assemblies. a

De´partement de Chimie, Universite´ de Montreál, Montreál, Que´bec H3C 3J7, Canada. E-mail: [email protected]; Fax: +1 (514) 340-3721; Tel: +1 (514) 340-5178 b CEA-Grenoble, INAC/SPrAM, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France w Permanent address: De´partement de geńie physique and Regroupement que´bećois sur les mate´riaux de pointe (RQMP), Ećole Polytechnique de Montreál, Montreál, Que´bec H3C 3A7, Canada. E-mail: [email protected]; Fax: +1 (514) 340-5195; Tel: +1 (514) 340-4711 Ext. 3901.

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The Royal Society of Chemistry 2010

We estimated the energies of adsorption of benzene (1a) and two azines, pyridine (2a) and 1,3,5-triazine (3a), as well as those of a series of related amines, including aniline (1b), 2-aminopyridine (2b), 2-amino-1,3,5-triazine (3b), 2,4-diamino1,3,5-triazine (3c), melamine (3d), 2,4,6-tris(dimethylamino)1,3,5-triazine (3e), and a related model compound, 2,4,6-trimethyl-1,3,5-triazine (3f). Calculations of electronic structure were carried out as discussed previously,3 initially using a method based on the local density approximation (LDA) of density functional theory (DFT) as implemented in the NWChem package.4 In the calculations, we used a fixed graphene sheet consisting of 204 atoms of carbon and 40 peripheral atoms of hydrogen, with C–C and C–H bond distances set at 1.46 A˚ and 1.01 A˚, respectively. The basis sets used for atoms of carbon and hydrogen in the graphene model were STO-3G and 6-31G*, respectively. For the adsorbates, we used the STO-3G basis set for atoms of carbon and 6-31G* for atoms of hydrogen and nitrogen. The molecular structures were fully optimized by the quasi-Newton method until a gradient convergence factor better than 10 5 hartree/bohr was reached. During the optimization steps, all species except the graphene sheet were free to move. The bond dissociation energies of the complexes were calculated with respect to the appropriate ground state species asymptote. In a subsequent step in our calculations, we evaluated the contribution of dispersive interactions, which are not formally taken into account within LDA limits. This was achieved by using the PBE functional to perform single-point calculations on LDA-optimized geometries,5 in conjunction with semiempirical dispersion corrections (D) developed by Grimme.6 Such calculations, which take into account dispersion and corrections, will be described as being at the PBE+D level.

The results of calculations at the LDA and PBE+D levels are summarized in Table 1. As previously noted,3 the adsorption of benzene (1a) estimated at the LDA level is relatively weak (4.5 kcal mol 1), indicating a physisorbed state. In contrast, the calculated value at the PBE+D level (13.5 kcal mol 1) is significantly larger and agrees well with the experimental value (13.6 kcal mol 1).7 Replacing atoms of carbon by nitrogen raises the energy of adsorption on graphene linearly, as Chem. Commun., 2010, 46, 2923–2925 | 2923

Table 1 Calculated energy of adsorption of benzene (1a), pyridine (2a), 1,3,5-triazine (3a), amino-substituted derivatives (1b, 2b, and 3b–e), and related compound 3f on graphene Adsorption energy/ kcal mol 1 Molecule

LDA

PBE+D

benzene (1a) pyridine (2a) 1,3,5-triazine (3a) aniline (1b) 2-aminopyridine (2b) 2-amino-1,3,5-triazine (3b) 2,4-diamino-1,3,5-triazine (3c) melamine (3d) 2,4,6-tris(dimethylamino)-1,3,5-triazine (3e) 2,4,6-trimethyl-1,3,5-triazine (3f)

4.5 5.9 9.7 14.0 14.0 16.4 22.6 25.1 29.7 18.2

13.5 13.9 14.4 21.3 20.8 23.1 31.5 36.0 43.9 20.3

demonstrated by the lower line in Fig. 1. These alterations reduce electron density on the remaining carbon atoms and thereby attenuate p–p repulsion between the adsorbate and graphene, leading to stronger binding. However, the overall increase from benzene (1a) to 1,3,5-triazine (3a) is modest (5.2 kcal mol 1) at the LDA level and becomes almost negligible (B1 kcal mol 1) once dispersive interactions are taken into account. Differences in adsorption energies calculated at the LDA and PBE+D levels can therefore be correlated with differences in dispersion energies, which decrease from benzene (1a) to 1,3,5-triazine (3a) as the p-electron density decreases. As a result, structural alterations in which atoms of carbon are replaced by nitrogen are not sufficient by themselves to produce molecules that bind tightly. In contrast, the energy of adsorption increases dramatically when NH2 groups are added to the aromatic core, as shown in Table 1 and Fig. 1. The upper line in Fig. 1 represents a linear extrapolation of the effect of adding one or two NH2 groups to the 1,3,5-triazine core. The small deviation of melamine (3d) from the linear extrapolation may simply reflect increasing p–p repulsion as the adsorbate approaches the graphene surface, driven by specific medium-range interactions involving the NH2 groups. Even so, the adsorption of melamine (3d) is stronger than that of trimesic acid,3 which has a high calculated affinity for graphene at the LDA level (22.6 kcal mol 1) and

Fig. 1 Plots of the energy of adsorption calculated at the PBE+D level as a function of the number of atoms of nitrogen in the aromatic ring (squares) and as a function of the number of NH2 groups added to a triazine core (circles). For comparison, values are also plotted for triazines 3e and 3f (triangles).

has been shown experimentally to form well-ordered assemblies on graphite.3,8 The inherently strong adsorption of melamine (3d), combined with the ability of aminotriazines to form planar hydrogen-bonded networks,9 explains its ability to generate robust ordered adlayers on graphite.10 Close examination of the geometry of adsorption of 2-amino-1,3,5-triazine (3b) reveals that the triazine core is slightly tilted, and the atoms of hydrogen in the NH2 group are directed toward the surface (Fig. 2a). These structural features suggest the existence of a significant attractive interaction between the NH2 group and graphene, which is sufficient to distort nitrogen from the planar trigonal geometry it normally favors in arylamines. The calculated distance from the surface to the nitrogen atom of the NH2 group (2.82 A˚) is smaller than that for the atoms of the core of unsubstituted benzene (1a) and 1,3,5-triazine (3a), which are 3.20 A˚ and 2.85 A˚, respectively. The decreased distance raises p–p repulsion, but the effect is minimized by tilting, and specific stabilization provided by the interaction of the NH2 group leads to strong overall adsorption. Similar geometric features are observed when 2,4-diamino-1,3,5-triazine (3c) and melamine (3d) are

Fig. 2 Views of the calculated structures of 2-amino-1,3,5-triazine (3b), 2,4-diamino-1,3,5-triazine (3c), and melamine (3d) adsorbed on a graphene sheet, showing how the NH2 groups are directed toward the underlying surface.

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Table 2 Mulliken electron population and adsorption energy (PBE+D) for triazines on graphene Molecule 1,3,5-triazine (3a) melamine (3d) 2,4,6-tris(dimethylamino)1,3,5-triazine (3e) 2,4,6-trimethyl-1,3,5-triazine (3f)

Net Mulliken charge |e|

Adsorption energy/kcal mol

0.14 0.34 0.36

14.4 36.0 43.9

0.18

20.3

1

adsorbed on graphene (Fig. 2b–c). The observed geometries underscore the importance of atoms at the points of intermolecular contact in assessing the adsorption of aromatic substrates on graphite.11 The energies and geometries of the adsorption of aminotriazines on graphene are directly related to the ability of the molecules to accommodate additional electron density arising from charge transfer contributed by the graphene sheet. As summarized in Table 2, the amount of net Mulliken electron population on adsorbed 1,3,5-triazines is correlated with their energies of adsorption. Charge transfer from graphene to aminotriazines occurs in part through the presence of hydrogen atoms in the substituents, which explains why terminal hydrogen atoms are directed toward the graphene surface. Related variations in the magnitude of charge transfer as a function of the structure of bound molecules may have useful applications, such as in devising non-invasive methods for hole-doping carbon-based nanomaterials.12 Comparison of the behavior of melamine (3d) with that of trimethyltriazine 3f, a close structural analog, is informative. The ability of melamine (3d) to bind much more strongly and accept more charge reflects the higher electronegativity of nitrogen, which decreases the net charge on the hydrogen atoms of the NH2 groups and thereby allows them to accept additional density contributed by graphene. This effect is magnified by NMe2 substituents, which provide a larger number of hydrogen atoms for interaction with the surface. In addition, these hydrogen atoms can be directed toward the surface without requiring substantial pyramidalization of nitrogen. Indeed, the sum of the bond angles around nitrogen in the NH2 groups of melamine (3d) is only 334.91, whereas that in the NMe2 groups of analog 3e (359.91) is close to the ideal sp2 value (3601). Our calculations suggest that aminotriazines are strongly adsorbed on graphite, in part through specific interactions of NR2 groups with the underlying surface. These interactions require distortions of NH2 groups out of the triazine plane, which may partially compromise their ability to participate in normal intermolecular hydrogen bonds. Nevertheless, adsorbed melamine (3d) engages in patterns of association closely similar to those seen in other phases.10 By offering a combination of unusually strong adsorption and predictable interadsorbate

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hydrogen bonding, aminotriazines stand out as particularly attractive substrates for creating 2D molecular assemblies on graphite, as confirmed by experimental studies in progress. We are grateful to the Natural Sciences and Engineering Research Council of Canada, the Ministe`re de l’E´ducation du Que´bec, the Canada Foundation for Innovation, the Canada Research Chairs Program, and the Universite´ de Montreál for financial support. We thank the Re´seau que´bećois de calcul de haute performance (RQCHP) for providing computational resources, and we are grateful to Dr Niri Govind at the Pacific Northwest National Laboratory for his support and advice related to DFT functionals.

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